Inelastic scattering investigations of vibrational and lattice interactions of the energetic material RDX

This dissertation investigates the molecular vibrational and lattice interactions of single crystals of cyclotrimethylene trinitramine (RDX), an explosive compound used extensively in military applications. The stiffness tensor measured through Brillouin scattering studies on the acoustic phonons indicate that RDX is most compliant along the crystallographic c axis and least compliant along the crystallographic a axis. Calculation of the linear compressibility using the compliance tensor indicates that single crystals of RDX exhibit nearly isotropic compressibility in all three crystallographic planes. This is consistent with reported hydrostatic compression studies on RDX. The molecular vibrations of RDX have been characterized using single-crystal, polarized Raman scattering. The six elements of the scattering tensor for the orthorhombic crystal system were calculated to reveal four different modal symmetries. The RDX molecule is distorted in the lattice to Cs symmetry, with molecules sitting at C1 sites in the D2h unit cell. This arrangement effectively destroys the expected modal symmetries; all molecular vibrational modes should be seen in all of the elements of the scattering tensor. A total of 50 separate vibrations were identified in the molecular region of the Raman spectrum. Analysis revealed that 20 of the vibrations arose from vibrational fundamentals and the remaining 30 arose from overtones and combination bands. The existence of certain overtones and combination bands, along with published RDX solution spectra, confirm the existence of fundamental molecular vibrations at low energy shifts. In addition, the number of combination bands and overtones suggest that many of the fundamental modes are highly anharmonic in nature. The optical phonons of RDX were also characterized using polarized, single-crystal Raman scattering. A total of 24 lattice phonon modes were found by probing the four different modal symmetries. The phonon located at 35 cm−1 in the B2g symmetry has not been previously reported. Piezomodulated Raman scattering studies on single-crystals of RDX showed the lattice modes and several molecular modes to be anharmonic in nature. Coupling between vibrational modes is clearly present, but unfavorable signal to noise ratios prevented quantitative assessment of their anharmonicity.